1. Field of the Invention
The present invention relates to a calendering apparatus for a paper making process. More particularly, the present invention relates to a calendering apparatus used for improving the surface properties of a paper sheet by applying a smoothness, a gloss or the like to a surface of the paper sheet.
2. Description of the Related Art
Many types of calendering apparatuses are used in a paper making process; typical of which are a machine calender, i.e., a chilled nip calender, and a supercalender.
The machine calender is arranged directly down stream of a drying part in a paper making machine, i.e., in an on-line arrangement to improve the surface properties of the paper sheet, and includes at least a nip formed by two chilled rolls made of a steel.
In the supercalender the chilled roll is arranged in a vertical direction alternately with an elastic roll, and the supercalender is used as an apparatus independent of the paper making machine, i.e., in an off-line arrangement. The supercalender forms several nips through which paper passes under a high pressure to produce a paper sheet having a superior smoothness, such as a gravure printing paper.
Further, a soft nip calender apparatus including a nip formed by the elastic roll and the chilled roll as a pair has been developed, and is used in an on-line arrangement like the machine calender.
The supercalender is usually driven off-line at a low speed, due to the need to take into consideration a possible deterioration of the elastic roll when operated under high pressure and multiple nips conditions. Therefore, in a calendering treatment of the paper sheet, the machine calender or the soft nip calender is most widely used.
The function of the soft nip calender using the elastic roll is clearly different from that of the machine calender with regard to improving the surface properties of a paper sheet, and this difference will be explained with reference to FIGS. 1 and 2. In those figures, a change in the surface of the paper sheet caused by a treatment with the machine calender is illustrated by a conventional cross section in a widthwise direction perpendicular to a running direction of the paper sheet in FIG. 1, and a change caused by the conventional soft nip calender is illustrated in FIG. 2, in the same manner as in FIG. 1.
A cross section in the widthwise direction of the paper sheet before the calendering treatment is illustrated in FIG. 1(A) and FIG. 2(A) respectively. As can be seen in FIG. 1(A) and FIG. 2(A), a thickness of the paper sheet just after a drying process is irregular; i.e., the paper sheet 10, 10' has thick portions 10a, 10a' and thin portions 10b, 10b'. A cross sectional shape of the paper sheet 10 while being nipped by two chilled rolls 50 of the machine calender is illustrated in FIG. 1(B), and a cross sectional shape of the paper sheet 10, 10' while being nipped by a chilled roll elastic roll 51 of the soft nip calender is illustrated in FIG. 2(B). In the former case, a surface layer of the thick portion 10a of the paper sheet 10 is made smooth by a pressing action of the chilled roll, but since a surface layer of the thin portion 10b of the paper sheet 10 does not come into contact with the chilled roll 50, small untreated portions remain on the paper sheet 10. In the latter case, when the paper sheet 10' is pressed with the chilled roll 50 and the elastic roll 51, a surface 11a opposite to the chilled roll 50 of the paper sheet 10' is made flat by the even cylindrical surface of the chilled roll 50, and the irregular shape of the surface 11a before the calendering operation is transferred to and duplicated at another surface 11b of the paper sheet 10'. The elastic roll 51 is deformed in accordance with the duplicated irregular shape of the surface 11b, due to its own elasticity, and accordingly, the surface 11a is made and remains smooth during the first stage of the calendering treatment and has a required gloss. The paper sheet 10' is then again treated by another calendering unit in which the chilled roll and the elastic roll are arranged in a reversed relationship, and thus the surface 11b is made flat. Accordingly, both surfaces 11a and 11b are then smooth and have the desired gloss.
The cross sectional shapes of the paper sheets after receiving the calendering treatment are illustrated in FIGS. 1(C) and 2(C). As can be seen from FIG. 1(C), the paper sheet treated with the machine calender has a substantially uniform thickness but the density and surface properties thereof are irregular. Further as can be seen from FIG. 2(C), the paper sheet treated with the soft nip calender has an irregular thickness but the density is constant, and thus the surface properties are improved in that the surface smoothness is uniform. Portions 12, 12' which are affected by the calendering treatment of the surface layer of the paper sheet in FIG. 1(C) and FIG. 2(C) are shown by double-hatching.
The surface of the paper sheet is made smoother by the soft nip calender, and therefore, the printability of the paper sheet treated with the soft nip calender is, superior to that of the paper sheet treated with the machine calender.
Nevertheless the soft nip calender has the following drawbacks. First the elastic roll of the soft nip calender is constructed by a layer of a synthetic resin having a high thermal resistance covering a metallic roll. The synthetic resin layer has an extremely low conductivity and a large coefficient of thermal expansion compared with the surface of the metal roll. Therefore, when the paper sheet in which a distribution of a water content and/or a weight per unit area of the paper sheet is irregular, or in which small fiber blocks are included, is treated with the soft nip calender, heat is retained by portions of the elastic roll in contact with thick portions of the paper sheet, and accordingly problems such as peeling at a boundary between the metallic roll and the synthetic resin layer, or the elastic roll becomes useless because the heated portion of the synthetic resin layer cannot resume its original shape, due to heat stress hysteresis, will occur.
Note that the calendering treatment should therefore not be carried out at a high temperature and a high pressure, as this will shorten the working life of the elastic roll. In particular, when the elastic roll is used on-line in an apparatus in which the running speed of the paper sheet is very high, the condition of the elastic roll must be closely observed at all times.
Consequentially, although the use of the elastic roll provides a superior effect, when the calendering apparatus including the elastic roll is continuously used in an actual industrial operation, often the use of the elastic roll does not provide a satisfactory calendering treatment capable of producing a paper sheet having a desired quality.